In-situ characterization of materials in various environmental conditions is a vitally informative technique in fields such as electrochemical energy storage, nanoparticle synthesis, and oxide thin film catalysis. In-situ characterization provides valuable information which is generally inaccessible to ex-situ experimentation, often clarifying phenomena which cannot be otherwise investigated. For electrodeposition processes specifically, it is valuable to know precisely the nature of the structure evolution of electrodeposited materials under electrochemically controlled conditions.
There is a great deal of literatures on electrochemical cell designs for various techniques including X-ray reflectivity and diffraction, high-energy X-ray scattering, and X-ray absorption spectroscopy. The reported in-situ X-ray characterization of thin amorphous water oxidation catalytic films is, to this point, only based on X-ray absorption. This method has typically been employed as an indirect probe of reaction surface because the formation of O2 bubbles on working electrodes reduces the total counts of fluorescence signal. In addition, X-ray absorption spectroscopy cannot monitor film growth during electrolysis because discerning between the same elemental species in the electrolyte and film is impractical. High-resolution X-ray reflectivity and diffraction usually utilize hard X-rays (<30 keV), which is able to obtain structural information of amorphous materials but q range is too short to get well resolved pair distribution function. X-ray optics in this energy regime allow relatively straight-forward investigation of the top surface of the electrode or solid/liquid interface, but these techniques do not seriously take into account background subtraction, which is a critical factor in transforming scattering pattern to pair distribution function (PDF) for amorphous materials. Hard X-ray scattering provides high-quality scattering patterns from amorphous thin metal oxide films, but the range of momentum transfer (q=kf−ki, where ki and kf are the incoming and outgoing wave vectors, respectively) is typically too short (<15 Å−1) to get sufficiently resolved pair distribution function (PDF).
The beam energy of X-ray absorption spectroscopy (XAFS)—including X-ray near edge structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS)—which is sensitive to the oxidation state and local coordination of element with short-range order, is close to X-ray absorption edge and could have strong potential to affect the measured results. X-ray beam-induced effect can be minimized by using high energy X-ray absorption. Scattering data from high-energy X-ray experiment will be converted to PDF to obtain the medium range of atomic pair distances which EXAFS cannot reach. PDF contributes immensely to the construction of structural information in complex and non-crystalline materials. Ultimately, however, both PDF and EXAFS are indispensable in understanding oxidation states and structural behavior during in-situ experiments. Here, we focus high energy X-ray scattering (HEXS) measurements, but the result of EXAFS measurement will be addressed as well. Up to now, the PDF technique has not been applied for the characterization of in-situ, electrode-supported amorphous thin metal oxide films (<2 μm) because of the need for a macroscopic scattering length (˜1 mm) and the limited X-ray focus (≥10 μm) of the current high-energy X-ray technique.